CRM 2011 - Required field based on other field's result

Hello everyone.

This is a quick and easy question, I think... how can I define a field in a form to set required or optional, based on other field's value.

For example: suppose that there are two fields in a form: Status and Description

1. If Status = Completed ---> Description is optional

2. If Status = Canceled ----> Description is required

Thanks in advance.

Regards, Andres.